In the manufacture of semiconductor memory devices and liquid crystal devices by photolithographic techniques, the method of transferring a mask pattern onto a substrate has been generally used. In this case, the illuminating light for exposure purposes, e.g., ultraviolet light is irradiated on the substrate having a photosensitive resist layer formed on its surface through a mask formed with a mask pattern and thus the mask pattern is photographically transferred onto the substrate.
The common type of the fine mask patterns for semiconductor memory devices, liquid crystal devices, etc., can be considered as regular grating patterns which are vertically or laterally arranged at equal intervals. In other words, in the mask pattern of this type the most dense pattern area is formed with a grating pattern composed of equally-spaced transparent and opaque lines which are alternately arranged in the X-direction and/or the Y-direction to realize the minimum possible line width which can be formed on the substrate and the other area is formed with a pattern of a comparatively low degree of fineness. Also, in any case any oblique pattern is exceptional.
Further, the ordinary photosensitive resist material has a non-linear light response characteristic so that the application of a light quantity greater than a certain level causes chemical changes to proceed rapidly, whereas practically the chemical changes do not progress when the quantity of light received is less than this level. As a result, there is a background that with the projected image of the mask pattern on the substrate, if the difference in light quantity between the light and dark portions is ensured satisfactorily, even if the contrast of the boundary between the light and dark portions is low more or less, the desired resist image as the mask pattern can be obtained.
With the recent tendency toward finer pattern structures for semiconductor memories and liquid crystal devices, projection exposure apparatus such as a stepper for transferring a mask pattern onto a substrate by reduction projection have been used frequently and a special ultraviolet light which is shorter in wavelength and narrow in wavelengh distribution range has also come into use as an exposure illuminating light. In this case, the reason for reducing the wavelength distribution range resides in eliminating any deterioration in the image quality of a projected image due to the chromatic aberrations of the projection optical system in the exposure apparatus and the reason for selecting the shorter wavelength is to enhance the contrast of the projected image. However, the actual situation is such that this attempt of reducing the wavelength of an illuminating light has reached the limit with respect to the requirements for finer mask patterns, e.g., the projection exposure of line width of the sub-micron order due to the non-existence of any suitable light source, the restrictions to lens materials and resist materials, etc.
In the case of such a finer mask pattern, the required value for the resolution (line width) of the pattern approaches the wavelength of the illuminating light so that the effect of the diffracted light produced by the transmission of the illuminating light through the mask pattern cannot be ignored and it is difficult to ensure a satisfactory light-and-dark contrast of the projected mask pattern image on the substrate, thereby particularly deteriorating the light-and-dark contrast of the line edges of the pattern.
In other words, while the diffracted beams of the 0-order, .+-.first-orders, .+-.second-orders and higher-orders produced at various points on the mask pattern by the illuminating light incident on the mask from above are respectively reconverged at the corresponding conjugate points on the substrate for imaging through the projection optical system, the diffracted beams of the .+-.first-orders, .+-.second-orders and higher-orders are further increased in diffraction angle as compared with the diffracted beam of the 0-order and are incident on the substrate at smaller angles for the finer mask pattern. This gives rise to a problem that the focus depth of the projected image is decreased greatly and a sufficient exposure energy is supplied only to a portion of the thickness of the resist layer.
As a measure for coping with such decrease in the focus depth, Japanese Laid-Open Patent Application No. 2-50417 (laid open on Feb. 20, 1990) discloses the method of arranging an aperture stop concentrically with the optical axis of each of an illumination optical system and a projection optical system to restrict the angles of incidence of an illuminating light on a mask and adjusting the opening diameters of the aperture stops in accordance with a mask pattern. This ensures the focus depth while maintaining the light-and-dark contrast of a projected image on a sample substrate. Even in the case of this known method, however, the diffraction angles of diffracted beams of the .+-.first-orders and higher-orders are still large as compared with a 0-order diffracted beam reaching substantially vertically to the surface of a substrate and practically all of them come out of the field of view of a projection lens, thereby producing on the substrate a projected mask pattern image composed by substantially only the 0-order beam component and having a weak contrast.
Also, while, in this case, there is the possibility of a part of the .+-.first-order diffracted beams coming within the field of view of the projection lens and reaching the substrate, in contrast to the 0-order diffracted beam incident substantially vertically on the substrate, the part of the .+-.first-order diffracted beams is incident on the substrate at a smaller angle and therefore it is pointed out that a satisfactory focus depth is still not obtainable.
On the other hand, U.S. Pat. No. 4,947,413 granted to T. E. Jewell et all discloses a lithography system in which an off-axis illumination light source is used and an interference of the 0-order diffracted beam and only one of the .+-.first-order beams from a mask pattern is made possible by use of a spatial filter processing in the Fourier transform plane within a projection optical system, thereby forming a high-contrast projected pattern image on the substrate with a high degree of resolution. With this system, however, the illumination light source must be arranged in an off-axis position in which it is obliquely directed to the mask, and also due to the fact that the 0-order diffracted beam and only one of the .+-.first-order diffracted beams are simply caused to interfere with each other, the dark-and-light contrast of the edges in the pattern image resulting from the interference is still unsatisfactory due to the unbalanced light quantity difference between the 0-order diffracted beam and the first-order diffracted beam.